Microwave-assisted reduction of acetophenones using Ni-Al alloy in water

Article abstract of DOI:10.1246/bcsj.79.791

The reduction of acetophenones using a Ni-Al alloy catalyst in water (H₂O and D₂O) under microwave irradiation proceeded to give the corresponding (deuterated) alkylbenzenes in good yields.

Full text of DOI:10.1246/bcsj.79.791

Short Articles
Bull. Chem. Soc. Jpn. Vol. 79, No. 5, 791–792 (2006)
791
O
Microwave-Assisted Reduction
of Acetophenones Using
Ni–Al Alloy in Water
Ni-Al alloy
water
1
20 °C, 60 min
oil bath heating 84 %
microwave heating 99 %
Akira Miyazawa,Ã1 Masashi Tashiro,1
G. K. Surya Prakash, and George A. Olah
2
2
Scheme 1.
1National Institute of Advanced Industrial Science and
Technology (AIST), AIST Tsukuba Central 5,
1
00
1
-1-1 Higashi, Tsukuba 305-8565
8
6
0
0
2Loker Hydrocarbon Research Institute,
University of Southern California,
Los Angeles, California 90089-1661, U. S. A.
Received October 19, 2005; E-mail: a.miyazawa@aist.go.jp
40
2
0
0
The reduction of acetophenones using a Ni–Al alloy
catalyst in water (H2O and D2O) under microwave irradia-
tion proceeded to give the corresponding (deuterated) alkyl-
benzenes in good yields.
0
20
40
60
80
100
120
reaction time / min
Fig. 1. Plots of yield versus reaction time in the reduction
of acetophenone. Reaction conditions are noted in
Utilization of water as a chemical reagent such as a hydro-
gen and oxygen source or an alternative to organic solvents is
Scheme 1.
1-phenylethanol, oil bath heating;
ethylbenzene, oil bath heating;
-phenylethanol, microwave heating;
benzene, microwave heating.
1
an essential aspect of ‘‘Green Chemistry.’’ The reductions of
carbonyl compounds is a fundamental organic transformation
1
ethyl-
2,3
in synthetic organic chemistry. The Clemmensen and the
4
Wolf-Kishners reductions are convenient and frequently used
protocols to selectively reduce carbonyl groups. However, the
former protocol cannot be applied to base sensitive substrates
and a high reaction temperature is required, while the latter is
conducted under strongly acidic conditions and occasionally
uses poisonous mercury. Very recently, we reported that the
reduction of acetophenones carried out in water in the presence
acetophenone was completely consumed within 60 min and
was converted to ethylbenzene in an almost quantitative yield.
The reaction rate was fast compared to heating with a conven-
tional oil bath heating. Figure 1 shows the reaction profiles for
the formation of ethylbenzene and the intermediate 1-phenyl-
ethanol. In the initial stage of the reaction, both conditions
(microwave irradiation and oil bath heating) had similar 1-
phenylethanol formation rates. After the yields of 1-phenyl-
ethanol reached 40%, microwave heating decreased the rate
of 1-phenylethanol faster than heating with the oil bath. It ap-
pears that the rates of hydrogen addition to the carbonyl group
are nearly the same, but that microwave irradiation accelerates
the hydrogenolysis step from 1-phenylethanol to ethylbenzene.
To determine the effect of the microwave irradiation, we in-
vestigated the reaction of a Ni–Al alloy with water. When the
5
of Ni–Al alloy gives the corresponding alkylbenzene in good
6
yields. Our method for the reduction of carbonyl groups is
simple and safe since organic solvents and a hydrogen gas cyl-
inder are unnecessary. On the other hand, this decade has seen
a tremendous increase in the number of reports on microwave-
assisted chemistry.7–9 Microwave irradiation drastically accel-
erates the reaction rate and occasionally enhances the selectiv-
ity. Thus, microwave irradiation should be a powerful tool in
the field of Green Chemistry. To achieve more environmental-
ly benign chemical processes, we have introduced microwave
irradiation into our reduction system. Herein, we wish to report
the microwave-assisted reductions of acetophenones with a
Ni–Al alloy in water.
ꢀ
suspension of Ni–Al alloy in water was heated at 120 C for
10 min using a preheated oil bath or microwave irradiation, the
resulting hydrogen pressure in each ampoule was 165 and
207 kPa, respectively (Scheme 2). These results indicate that
microwave irradiation directly influences hydrogen generation
in the reaction of the Ni–Al alloy with water. Microwaves col-
lide with the Ni–Al alloy and cause hydrogen to be rapidly
The reactions were conducted in the presence of a Ni–Al
ꢀ
alloy in water at 120 C using a preheated oil bath or micro-
wave irradiation in a pressure resistant glass ampoule. During
the reaction, the temperature of the ampoule was maintained at
ꢀ
generated since aluminum is a good microwave absorber that
10,11
1
20 C by varying the irradiation power of the microwaves
reacts with water to generate hydrogen.
Thus, this rapid
between 10 and 100 W. Scheme 1 shows the preliminary re-
sults using both heating methods. Upon microwave irradiation,
generation of hydrogen seemed to be a key step of the accel-
eration of acetophenone reduction.
Published on the web May 12, 2006; DOI: 10.1246/bcsj.79.791

7
92 Bull. Chem. Soc. Jpn. Vol. 79, No. 5 (2006)
Ó 2006 The Chemical Society of Japan
Table 1. Preliminary Results of Acetophenon and Propio-
phenon Reduction with Ni–Al Alloy in Water (H2O and
D2O)
Ni-Al alloy
H O
2
H2
a)
1
1
20 °C
0 min
O
Ni-Al alloy
H pressure
R
R
2
water
oil bath heating
microwave heating
165 kPa
207 kPa
microwave (50 W)
Entry
R
Water
Time/min
Yield/%
Scheme 2.
1
2
3
4
Me
Et
Me
Et
H2O
H2O
10
10
30
30
quant
81
quant
quant
6
4
2
0
0
0
0
D O
2
D2O
a) Conditions: starting ketones (200 mg), Ni–Al alloy (1.0 g),
and water (2 mL) were used.
should be very useful since the reaction proceeds very rapidly
and can be used as a deuteration.
Experimental
The Ni–Al alloy (50 wt % of Ni and Al, Wako Chemical Ltd.)
was used as received. Other chemicals purchased were used as
received without further purification. D2O (99% D-content) was
used for the deuteration reaction. The microwave apparatus
0
25
50
75
100
125
150
irradiation power / W
Fig. 2. Plots of conversion against irradiation power in the
reaction of Ni–Al alloy and water.
(
CEM Discover, 2.45 GHz frequency, CEM Corporation, NC
U.S.A.) was used for irradiation. GC analyses were performed us-
ing a Shimadzu GC-14A with a capillary column (DB-1, 50 m).
The GC-MS measurements were conducted with a Nippon Denshi
SUN-200 spectrometer (electron impact system with a 70 eV ion-
ization energy).
A typical microwave irradiation experiment (i.e. Scheme 1)
used 200 mg (1.67 mmol) of acetophenone, 1.0 g of Ni–Al alloy,
and 2 mL of H2O in a pressure resistant glass ampoule equipped
with a rubber septum and an aluminum cap. The loaded ampoule
As previously mentioned, to generate isothermal reaction
conditions, the irradiation power was automatically fluctuated.
Therefore, we investigated this microwave-assisted reduction
under the conditions at constant power with continuous irradi-
ation in more detail.
Figure 2 plots the microwave irradiation power against the
acetophenone conversion. Although the acetophenone conver-
sion was increased as the microwave irradiation power increas-
ed up to 50 W, more powerful irradiation did not affect the
conversion. When the reaction of the Ni–Al alloy with water
was conducted with continuous microwave irradiation of 50 W
under the same conditions described in Scheme 2, the hydro-
gen pressure was 413 kPa.
(
10 mL capacity) was introduced into the cavity of the microwave
ꢀ
apparatus and irradiated with microwaves for 60 min at 120 C.
The ampoule was cooled in an ice/water bath and the organic ma-
terials were extracted with ether. The ether fraction was analyzed
by GC and GC-MS to identify and quantify the products.
Table 1 lists the results of acetophenone and propiophenone
reduction using the Ni–Al alloy catalyst in water (H2O and
D2O) under continuous microwave irradiation at 50 W. When
microwaves were continuously irradiated at a fixed power
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ꢀ
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H2O, the expected ethylbenzene-d5 and n-propylbenzene-d7
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In summary, we have demonstrated the convenient reduc-
tion of acetophenones using a Ni–Al alloy with water under
microwave irradiation. The presently developed method
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